Chemical Reactions And Equations

Chemical Change - objectives 1. List the common indicators of a chemical change and use them to identify when a chemical reaction has probably occurred. They are: color change, change of state (gas bubbles, precipitate, etc), temperature change, pressure change in a closed container, and change in odor. Note the emphasis on change

Indications of a Chemical Reaction There are some easily observed changes that usually indicate a chemical reaction Color change Change of state (gas bubbles, precipitate, etc), Temperature change Pressure change in a closed container Change in odor

1. Color change

2. Change of state: gas bubbles Release of gas bubbles when two substances mixed is often evidence of chemical reaction Antacid in water

2. Change of state: formation of precipitate Many reactions happen in solution If solid appears after two solutions mixed, a reaction has probably happened Example: Pb(NO3)2 + KI A bright yellow solid PbI2 appears as the proof that the reaction has happened

2. Change of state: formation of precipitate The solid that appears is called precipitate Precipitate  a solid that is made as a result of a chemical reaction in solution and that separates from the solution PbI2 precipitate

3. Temperature change Reaction between vinegar and baking soda is accompanied by drop of temperature Reaction between glycerin C3H5(OH)3 and potassium permanganate KMnO4 produces extensive heat (burning) Note: Heat by itself NOT necessarily sign of chemical change

4. Pressure change in a closed container If reaction is going in the gas phase and new gases are formed, the pressure in the closed container will change

5. Odor change Odor Changes It only takes one experience with a rotten egg to learn that they smell different that fresh eggs.  When eggs and food spoil, they undergo a chemical change.  The change in odor is a clue to the chemical change

Understanding Check Is it a chemical or physical change? “Dry ice”, solid carbon dioxide, is sublimated into gaseous CO2 at room temperature Salt is dissolved in water Iron rusts in a damp environment Gasoline burns in the presence of oxygen Liquid hydrogen peroxide (H2O2) decomposes to water and oxygen Rotting of eggs

Chemical Equations - objective 2. Given a word equation, write a chemical equation (this requires good formula writing skills)

Chemical Reactions and Equations Chemical reaction is a process by which one or more substances are changed into one or more different substances Chemical equations represent chemical reaction with symbols and formulas. Chemical equations show the identities, physical phases and relative amounts of the reactants and products in a chemical reaction Reactants (state) → Products (state) Reactants are compounds being brought together to react Products are the new compounds that are made in the reaction

Physical Phase Solid (s) Liquid (l) Gas (g) Aqueous solution (aq) – solution in water Examples: Na (s); O2 (g); HCl (l); Ca(NO3)2 (aq)

Word Equations First step in writing chemical equation is to identify facts to be represented Word equation  “equation” in which reactants and products are represented by words Does not give quantities of reactants/products Reactants ==> Products chemicals that react chemicals that are produced hydrogen + oxygen water hydrogen peroxide water + oxygen magnesium + oxygen magnesium oxide sodium chloride sodium + chlorine

Formula Equations Mg (s) + O2 (g)  MgO (s) Word equation: hydrogen (gas) + oxygen (gas)  water (liquid) Formula equation: H2 (g) + O2 (g)  H2O (l) magnesium (solid) + oxygen (gas)  magnesium oxide (solid) Mg (s) + O2 (g)  MgO (s)

Characteristics of Chemical Equations The equation must contain correct formulas for the reactants and products. The Law of Conservation of Mass must be satisfied: mass is neither created nor destroyed in any ordinary chemical reaction. The quantity of substances produced (products) by a chemical reaction is always equal to the quantity of the reacting substances (reactants). The “quantity” here means mass or the number of atoms for each element participating in reaction

Balancing a Chemical Equation Write a formula equation H2O(l)  H2(g) + O2(g) Create a table showing the number of atoms for each element: This equation is not balanced! The number of atoms Reactants Products H 2 O 1

Balancing Equations Correct the number of O by placing coefficient 2 in front of H2O 2H2O(l)  H2(g) + O2(g) Correct the table: Oxygen is now balance with 2 on left and 2 on right. H is not! Now balance hydrogen – 4 on left, 2 on right. Add coefficient 2 to H2 2H2O(l)  2H2(g) + O2(g) balanced! The number of atoms Reactants Products H 4 2 O The number of atoms Reactants Products H 4 O 2

Guidelines for Balancing Equations Create a table of elements for reactant and product side of the equation Balance the different types of atoms one at a time. First balance the atoms of elements that are combined and that appear only once on each side of the equation. Balance polyatomic ions that appear on both sides as single units. Balance lone atoms, especially H and O atoms (or any), last.

DO NOT!!!..... DO NOT WRITE INCORRECT FORMULAS, THIS WILL MESS UP YOUR BALANCING DO NOT CHANGE SUBSCRIPTS IN FORMULAS TO BALANCE THE EQUATION! H2O(l)  H2(g) + O2(g) H2O(l)  H2(g) + O(g)

DO!..... When you think you have balanced the equation, COUNT THE NUMBERS OF EACH TYPE OF ATOM ON EITHER SIDE OF THE EQUATION 2H2O(l)  2H2(g) + O2(g) Reactant side Product side H 4 O 2

Sample Problem 1 Write balanced chemical equations for the following reaction: Solid sodium combines with chlorine gas to produce solid sodium chloride. Word reaction: Sodium (solid) + chlorine (gas)  sodium chloride (s) Formula reaction: Na(s) + Cl2(g) → NaCl(s) Balanced reaction 2Na(s) + Cl2(g) → 2NaCl(s)

Balance Reaction Na(s) + Cl2(g) → NaCl(s) Count atoms: Na (in compound): 1 on the left, 1 on the right - balanced Cl (lone element): 2 on the left, 1 on the right – not balanced! Adjust Cl: Na(s) + Cl2(g) → 2 NaCl(s) Na: 1 on the left, 2 on the right – not balanced! Cl: 2 on the left, 2 on the right – balanced Adjust Na: 2 Na(s) + Cl2(g) → 2 NaCl(s) Count atoms – all balanced! Na: 2 on the left, 2 on the right Cl: 2 on the left, 2 on the right

Sample Problem 2 Write word, formula, and balanced chemical equations for magnesium and hydrochloric acid (HCl) react to produce magnesium chloride and hydrogen. Word equation: Magnesium (solid) + hydrochloric acid (liquid)  magnesium chloride (solid) + hydrogen (gas) Formula equation: Mg (s) + HCl (l)  MgCl2 (s) + H2 (g) Count atoms: Mg (in compound): 1 on the left, 1 on the right Cl (in compound): 1 on the left, 2 on the right H (lone element): 1 on the left, 2 on the right Adjust HCl: Mg + 2HCl  MgCl2 + H2 Recount the atoms – all balanced!

Sample Problem 3 Word equation: Methane (gas) + oxygen (gas)  carbon dioxide (gas) + water (gas) Formula equation: CH4(g) + O2(g)  CO2(g) + H2O(g) Reactants Products Balanced? 1. C (in compounds in both sides) 1 Yes 2. H (in compound) 4 2 No 3. O (in compound and a lone element) 3

Sample Problem 3 (cont.) Adjust H: CH4(g) + O2(g)  CO2(g) + 2H2O(g) Adjust O: CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) Reactants Products Balanced? 1. C (in compounds in both sides) 1 Yes 2. H (in compound) 4 3. O (in compound and a lone element) 2 No Reactants Products Balanced? 1. C (in compounds in both sides) 1 Yes 2. H (in compound) 4 3. O (in compound and a lone element)

Practice Problems Fe + Cl2 = FeCl3 2Fe + 3Cl2 = 2FeCl3 Zn + HCl  ZnCl2 + H2 Zn + 2HCl  ZnCl2 + H2 Al + O2  Al2O3 4Al + 3O2  2Al2O3